engleski

Influence of the Alkylammonium chain length and pH on the formation of Alkylammonium Cholate catanionic surfactants

The bile salts, as natural steroidal surfactants, play divergent roles in biology and in supramolecular chemistry. These compounds are characterized by four chained rings which form rigid body of the molecule, three OH groups on one side and two methyl groups on the opposite side, responsible for facial amphiphilicity of the molecules. This property is responsible for dominant mode of packing in crystal structures: the hydrophobic faces of the molecules join to make bilayers with hydrophobic surfaces. The crystal packing is realised through stacking of such bilayers. Because of the shape of molecules, such an assembly leaves open cavities on the hydrophobic interfaces between the bilayers, that can include various guest molecules. Here we report on the formation of oppositely charged catanionic surfactants alkylammonium cholates, formed form aqueous surfactant mixtures of an anionic sodium cholate with the positively charged n-alkylammonium chlorides, by varying the surfactant chain length from n = 10 to 16 carbon atoms. In these amphiphilic mixtures surfactants react electrostatically and strong synergism is found. The formation of needle-like colorless precipitates is observed in equimolar solution. Because the cholate headgroup has a carboxylic acid functionality (pKa = 5.0), by varying pH the morphology of precipitate and the contribution of complexes are continuously changed. In the neutral and slightly acidic medium the cationic : anionic surfactant molar ratio in the solid-state complex is detected as 1:1, at 5.4 > pH > 2.5 as 1:2, and from more acidic aqueous mixtures precipitates the cholic acid alone. To characterize and identify the different alkylammonium cholate products, Fourier transform infrared spectroscopy (FTIR), NMR, X-ray scattering of single crystals and elemental analysis are used. All of the alkylammonium cholates crystals belong to the monoclinic space group P 21. Unit cell of 1:1 complexes built four (Z = 4), and that of 1:2 complex six molecules (Z = 6). The increase of alkyl chain length results with increase of a and c cell lengths, and cell angle β . The b cell length is constant (approximately 7.9 Å ; ) and characteristic for this group of investigated compounds, independently also on Z. The structures of 1:1 complexes have similar crystal packings: the molecules of cholic acid form hydrogen bonds with each other thus creating characteristic layers with hydrophobic parts on the outside. Between those layers the long aliphatic parts of the alkylammonium are trapped. The charged NH3+ parts form various hydrogen bonds with OH and COO- groups of the cholic acid molecules and with crystal water in the hydrophilic interior (sandwich type structure). The 1:2 complex reveals quite different and novel crystal packing: the hydrophilic regions are separated into layers which are not parallel andcross each other forming hydrophobic channels which host the alkylammonium cations (crossing bilayers structure).

catanionic surfactants; bile salts

nije evidentirano